1. Field of the Invention
The present invention relates to the field of power converters of low voltage switched-mode power supply type. The present invention more specifically applies to isolated power supplies, that is, supplies that have no common point between the input voltage (for example, the A.C. supply system) and the regulated D.C. output voltage. The isolation is obtained by means of a transformer having a primary winding associated with a pulse-width modulation controlled switch, and having a secondary winding associated with a diode and with a capacitor providing the output voltage.
2. Discussion of the Related Art
FIG. 1 shows a conventional example of a switched-mode power supply of the type to which the present invention applies. Two input terminals P, N receive an A.C. voltage Vac, for example, the mains voltage. Voltage Vac undergoes a rectification, for example a fullwave rectification by means of a diode bridge 1. The A.C. input terminals of bridge 1 are connected to terminals P and N, and its rectified output terminals 2, 3 provide a voltage Vr. Voltage Vr is generally smoothed by means of a capacitor C1 connected between terminals 2 and 3 which form the input terminals of the actual switched-mode power supply.
The converter of FIG. 1 is a so-called flyback converter in which a transformer 4 with inverted phase points has its primary winding 5 connected in series with a switch 6 between terminals 2 and 3. The phase point of winding 5 is connected to a terminal of switch 6, the other terminal of which is connected to terminal 3. Switch 6 is connected in switched mode and at a non-audible high frequency (generally greater than 20 kHz). A secondary winding 7 of transformer 4 is associated with a capacitor C2 across the terminals Sp and Sn of which is provided D.C. output voltage Vout. The phase point of winding 7 is connected to terminal Sp by a diode D1, the cathode of diode D1 being connected to terminal Sp. The other terminal of winding 7 is connected to terminal Sn. Ground terminals 3 and Sn are isolated from each other by means of a capacitor Ci.
When switch 6 is on, the phase point of winding 7 is at a negative potential. Diode D1 thus is off and a current is stored in primary winding 5. Upon turning off of switch 6, the phase points of windings 5 and 7 both become positive. Diode D1 is forward biased. Capacitor C2 is then charged with the power transferred to secondary winding 7.
Switch 6 (for example, a MOS transistor) is, in the example of FIG. 1, integrated in a circuit 10 with its electronic control circuit. An example of such an integrated circuit, sold by STMicroelectronics Company, is known under trade name VIPER. Circuit VIPER is comprised of an input terminal Vdd intended for receiving a positive power supply, a voltage reference terminal Vss connected to ground 3, and a terminal OSC conditioning an oscillation frequency. Circuit 10 further includes a terminal COMP for compensating the regulation loop, connected by a resistor R5 in series with a capacitor C5 to ground 3. Finally, a terminal 12 is connected to the drain of the integrated N-channel transistor, the source of which is connected to terminal Vss. The gate of transistor 6 is connected at the output of a control circuit 11 (CTRL). Circuit 11 includes a comparator (not shown), a first input of which receives an internal voltage reference and a second input of which is connected, internally, to the positive supply terminal. The control, that is, the modification of the width of control pulses of switch 6, is performed by for example using a loop of regulation of integrated circuit 10, which attempts maintaining its supply voltage (Vdd-Vss). This type of regulation is performed on the primary side of the transformer. The output voltage may also be regulated based on a measurement on the secondary side, transferred to circuit 10 by a galvanic isolation element (for example, an optocoupler). Terminal OSC is connected to the midpoint of a series association of a resistor R3 and of a capacitor C3 between a line 13 of local supply of circuit 10 and the ground. Resistor R3 and capacitor C3 set the oscillation frequency. A capacitor C4 for filtering the local supply voltage is connected between terminal 13 and terminal 3.
A problem which arises in flyback-type converters is that, when a short-circuit occurs at the output of the power converter, diode D1 and transformer 4 are not protected. They thus risk being damaged under the effect of the strong current that flows through the transformer. Further, a melting of the transformer breaks the galvanic isolation, which is particularly dangerous if the power converter is supplied by the mains. Standards generally determine the short-circuit strength duration of a power converter of this type.
In an application to a switched-mode converter, local supply line 13 of circuit 10 is often connected, as illustrated in FIG. 1, by a diode D2, to the phase point of an auxiliary winding 8 of transformer 4. In this case, the other terminal of auxiliary winding 8 is connected to reference terminal 3 of the rectified voltage. Auxiliary winding 8 has the function of providing supply voltage Vdd of circuit 10. The output voltage then is set by the transformation ratio between auxiliary winding 8 and secondary winding 7. Auxiliary winding 8, which gives an image of the output voltage, is used, said winding being in direct phase relation with secondary winding 7.
In such an assembly, a current detector (not shown) may be provided in series with switch 6. The result of the detection is then provided to a comparator which, by means of adapted logic circuits, opens switch 6 when the current exceeds a threshold. The amount of power transmitted to the secondary is thus reduced. Further, since auxiliary winding 8 is in direct phase relation with secondary winding 7, the voltage drop which appears across the secondary winding is, after a time depending on the value of capacitor C4, seen by the supply line of circuit 10. The supply of circuit 10 becomes insufficient for its operation, which guarantees the turning off of switch 6.
FIGS. 2, 3A, 3B, and 3C illustrate the operation of such a power converter in normal state and in short-circuit at the secondary. FIG. 2 illustrates an example of the shape of voltage VAUX across auxiliary winding 8 in normal operation. FIGS. 3A, 3B, and 3C respectively show the course of voltage VAUX, of current 1 in switch 6 and of local supply voltage Vdd of circuit 10 when the secondary of the transformer is short-circuited.
In normal operation, upon each turning-off (times t1) of switch 6, voltage VAUX abruptly increases from a negative value to a demagnetization value VDEM. Value VDEM is only reached after a few spurious oscillations associated with the turning-off of switch 6. Once the demagnetization is complete, voltage VAUX drops (times t2) and exhibits oscillations centered on the voltage zero until the turning-on (times t3) of switch 6 where voltage VAUX becomes negative again. The same waveform is obtained across secondary winding 7, before the voltage is filtered by capacitor C2.
When a short-circuit is present between terminals Sp and Sn (FIGS. 3A to 3C), the current in switch 6 is limited to a value Imax (times t10 to t11) by the previously-described detection circuit. Supply voltage Vdd of circuit 10 then progressively decreases (FIG. 3C). This progressive decrease reaches a threshold (VddOFF) below which circuit 10 no longer receives the sufficient voltage. From this time t11 on, switch 6 remains off and no current is transmitted to the secondary. However, this restarts the starting circuit generally associated with the power converter. This restarting causes a progressive increase of local supply voltage Vdd. When this voltage reaches (time t12) the operating threshold (VddON) of circuit 10, high current surges occur again (times t12 to t13). Voltage VAUX (FIG. 3A) exhibits oscillations for each current peak.
The scale is different between FIG. 2 and FIGS. 3A to 3C. In FIG. 2, the switched mode period (on the order of 10 microseconds) of the supply voltage has been shown. In FIGS. 3A to 3B, the time interval separating times t10 and t11 during which the decrease of local supply voltage Vdd takes several switching cycles and lasts, for example, for approximately 100 milliseconds, has been shown.
To respect the standards, a duty ratio that enables the transformers to withstand the high currents Imax (FIG. 3B) which, even though they are limited, are much greater than nominal current Inom of normal converter operation. For example, for a nominal current of 2 amperes, the limiting current is on the order of 10 amperes.
A problem that remains in this conventional solution is that transformer 4 is still urged at its maximum power. Accordingly, the auxiliary winding generates spurious noise (FIG. 3A). This noise is also present in normal state but its amplitude is much greater for a short-circuit of the secondary. Accordingly, although the auxiliary voltage drops, amplitudes sufficient for an autonomous supply of the control circuit are often present.
Another problem is linked to the minimum conduction duration of switch 6. Indeed, it must be possible to turn off switch 6 sufficiently rapidly after each turning-on, otherwise the demagnetization under the low local supply voltage of the auxiliary circuit cannot be performed, and the current is then no longer controlled.